Recently, the demand for a special electric motor has been greatly increased in automobiles, medical equipment, bio-related industry, semiconductor devices, aerospace fields, and military fields. Especially, the demand for bearingless electric motors has been more greatly increased in application fields (e.g., a grinder of a machine tool and a turbo molecular pump) requiring an ultra-high speed rotation and medical equipment fields (e.g., a blood pump and a bio-pump) to which a mechanical bearing is not adaptable. Different from a support structure of a mechanical bearing in a typical electric motor, a bearingless electric motor simultaneously generates driving torque and radial force used for suspending a rotor through a uniform air-gap. The radial force is used to provide the uniform air-gap by controlling magnetic flux generated from stator windings without a bearing used in the typical electric motor. Different from an existing air bearing or magnetic bearing system, the bearingless electric motor is designed such that the radial force is generated from the stator winding. Accordingly, since the bearingless electric motor does not require an additional space to install a magnetic bearing or an air bearing, the size of the electric motor can be reduced, and a system can be simplified. Therefore, the bearingless electric motor has a significant economical advantage.
FIG. 1 is a view showing the structure of a conventional bearingless switched reluctance motor (BLSRM) equipped with torque windings and windings generating suspending force for a rotor of a phase.
An SRM rotates a rotor by using reluctance torque according to the variation of magnetic reluctance. The SRM is manufactured at a low cost, and the maintenance of the SRM is rarely required. In addition, since the SRM is operated with high reliability, the life time of SRM may be nearly permanent.
As shown in FIG. 1, a reference character Nma represents a main winding of a phase A to generate rotational torque, and reference characters Nsa1 and Nsa2 represent auxiliary windings of the phase A to generate radial force. As shown in FIG. 1, the main winding Nma is constructed by connecting four coils in series, and the auxiliary windings Nsa1 and Nsa2 are constructed by winding two coils in series, respectively. In the case of phases B and C, main windings and auxiliary windings are wound and arranged similarly to the phase A. As shown in FIG. 1, a bolded solid line represents a magnetic flux of each pole, which is generated by current flowing through the main winding, and a dashed line represents leakage flux generated by current ima flowing through the auxiliary winding Nsa1. The resultant flux generated by the current of the main winding and the auxiliary winding is increased in the Air-gap1, and decreased in the Air-gap 2.
Radial force in an α-direction may be generated due to difference in the resultant flux of each air-gap. Similarly, radial force in a β-direction may be generated due to resultant flux of the current ima and current isa2 flowing through the auxiliary winding Nsa2. The direction of such radial force for the rotor may be uniformly maintained by controlling the intensity and direction of current flowing through the auxiliary windings according to positions of the rotor.
However, in the BLSRM of FIG. 1, since magnetic flux to generate torque mutually interacts with magnetic flux to generate magnetic levitation force, the control of the two kinds of magnetic flux is difficult.
FIG. 2 is a graph showing the characteristics of torque and suspending force for a rotor in the typical BLSRM. A region to generate torque partially overlaps with a region to generate suspending force for the rotor in a pole structure for one phase due to the characteristics of the torque and the suspending force for the rotor in the typical BLSRM. In the overlap region, one of the torque and the suspending force for the rotor must be used.
Accordingly, since the use of the torque region is limited in order to generate stable suspending force for a rotor, torque ripples are increased, and operating efficiency is reduced.
As described above, the BLSRM has a simple mechanical structure and has no demagnetization characteristic of a permanent magnet for the radial force. On the contrary, since the conventional BLSRM has a configuration in which windings generating torque are arranged on stator poles together with windings generating radial force, significant interference may be caused between the windings generating the torque and the radial force.
In addition, since the radial force region overlaps with the torque region, the use of the torque region is limited in order to generate continuous radial force. Accordingly, the generation of constant torque may be difficult, and the efficiency may be largely reduced.